In a liquid crystal display device, a classification based on an operating mode for liquid crystal molecules includes a phase change (PC) mode, a twisted nematic (TN) mode, a super twisted nematic (STN) mode, an electrically controlled birefringence (ECB) mode, an optically compensated bend (OCB) mode, an in-plane switching (IPS) mode, a vertical alignment (VA) mode, a fringe field switching (FFS) mode and a field-induced photo-reactive alignment (FPA) mode. A classification based on a driving mode in the device includes a passive matrix (PM) and an active matrix (AM). The PM is classified into static, multiplex and so forth, and the AM is classified into a thin film transistor (TFT), a metal insulator metal (MIM) and so forth. The TFT is further classified into amorphous silicon and polycrystal silicon. The latter is classified into a high temperature type and a low temperature type based on a production process. A classification based on a light source includes a reflective type utilizing natural light, a transmissive type utilizing backlight and a transflective type utilizing both the natural light and the backlight.
The liquid crystal display device includes a liquid crystal composition having a nematic phase. The composition has suitable characteristics. An AM device having good characteristics can be obtained by improving characteristics of the composition. Table 1 below summarizes a relationship in two characteristics. The characteristics of the composition will be further described based on a commercially available AM device. A temperature range of the nematic phase relates to a temperature range in which the device can be used. A preferred maximum temperature of the nematic phase is about 70° C. or higher, and a preferred minimum temperature of the nematic phase is about −10° C. or lower. Viscosity of the composition relates to a response time of the device. A short response time is preferred for displaying moving images on the device. A shorter response time even by one millisecond is desirable. Accordingly, a small viscosity in the composition is preferred. However, the statement does not apply to a mode in which electric-field-induced transition is exhibited based on a Kerr effect (for example, a polymer-stabilized blue phase (PSBP) liquid crystal display, a nanocapsule liquid crystal display), and a higher speed response without depending on viscosity of a liquid crystal can be expected.
TABLE 1Characteristics of Composition and AM DeviceNo.Characteristics of compositionCharacteristics of AM device1Wide temperature range of aWide usable temperature rangenematic phase2Small viscosity1)Short response time3Suitable optical anisotropyLarge contrast ratio4Large positive or negativeLow threshold voltage anddielectric anisotropysmall electric power consumptionLarge contrast ratio5Large specific resistanceLarge voltage holding ratio andlarge contrast ratio6High stability to ultravioletLong service lifelight and heat7Large elastic constantLarge contrast ratio andshort response time1)A composition can be injected into a liquid crystal display device in a short time.
Optical anisotropy of the composition relates to a contrast ratio in the device. According to a mode of the device, large optical anisotropy or small optical anisotropy, more specifically, suitable optical anisotropy is required. A product (Δn× d) of the optical anisotropy (Δn) of the composition and a cell gap (d) in the device is designed so as to maximize the contrast ratio. A suitable value of the product depends on a type of the operating mode. In a device having a mode such as TN, a suitable value is about 0.45 micrometer. In the above case, a composition having the large optical anisotropy is preferred for a device having a small cell gap. Large dielectric anisotropy in the composition contributes to low threshold voltage, small electric power consumption and a large contrast ratio in the device. Accordingly, the large dielectric anisotropy is preferred. Large specific resistance in the composition contributes to a large voltage holding ratio and the large contrast ratio in the device. Accordingly, a composition having the large specific resistance at room temperature and also at a temperature close to the maximum temperature of the nematic phase in an initial stage is preferred. The composition having the large specific resistance at room temperature and also at a temperature close to the maximum temperature of the nematic phase even after the device has been used for a long period of time is preferred. Stability of the composition to ultraviolet light and heat relates to a service life of the liquid crystal display device. In the case where such stability is high, the device has a long service life. Such characteristics are preferred for an AM device used in a liquid crystal projector, a liquid crystal television and so forth.
A composition having positive dielectric anisotropy is used in an AM device having the TN mode. A composition having negative dielectric anisotropy is used in an AM device having the VA mode. In an AM device having the IPS mode or the FFS mode, a composition having positive or negative dielectric anisotropy is used. In an AM device having a polymer sustained alignment (PSA) mode, a composition having positive or negative dielectric anisotropy is used. Examples of a liquid crystal composition having positive dielectric anisotropy are disclosed in Patent literature Nos. 1 and 2 described below.